High efficient Radio Frequency (RF) power amplifiers are critical components in modern radio communications systems. With worldwide increasing use of radio communications means, the energy consumption of very large number of radio base stations deployed becomes significant, which not only increases the cost for radio network operators, but also causes environment problems.
During the past, numbers of highly efficient power amplifiers have been proposed in literature. In particular, the switching mode power amplifier, such as class D, class E, class S, etc., have received the increasing attention for their possible applications in both radio receivers and radio base stations due to their very high efficiency. However, these types of switching-mode power amplifiers are highly non-linear. Special circuit architecture, such as, LInear amplification using Non-linear Components (LINC) technique, are usually required for their use in highly linear RF power amplifier applications. Although recent ideas of applying digitally, sigma-delta, modulated signals directly to switching-mode power amplifiers are promising. The operating frequency of these switching mode power amplifiers is, however, so far limited and not ready to be used in current radio transmitters with GHz carrier wave frequencies. The problem is that the high efficiency of these power amplifiers cannot be maintained due to the upper frequency limit of available RF power transistors and their intrinsic Ohmic losses with the current semiconductor technology.
In U.S. Pat. No. 3,896,395 two NLPAs 104,106 are used to amplify constant envelope RF signals directly after the mixers 108, 110 for both the I and Q branches, as shown in FIG. 1. The I and Q baseband signals SI, SQ are converted into a bi-state (+1, −1) pulse train using a delta coder 112, 114. This will generate a constant envelope PSK modulated RF signal alter mixing with the Local Oscillator (LO) 116. However, a high level RF power combiner 102 used in the approach shown in this document may cause power lass (e.g., 3 dB loss), which win significantly reduce the total power efficiency of the linear amplification by Sampling Technique (LIST) as described in the document.
A drawback with the power amplifier disclosed U.S. Pat. No. 3,896,395, is that the power combiner 102 causes power loss as described above and hence degrade the efficiency.